At present, in the fields of medical monitoring, household appliances, consumer electronics, Internet of Things, intelligent sensing, automotive electronics, intelligent control and so on, the demand for integrated intelligent temperature sensors grows constantly, and the demand for the precision of the integrated intelligent temperature sensors becomes stricter. Domestic and overseas research institutions and enterprises get some research achievements in the aspect of the integrated intelligent temperature sensors, such as An All-Digital Smart Temperature Sensor with Auto-Calibration in 65 nm CMOS Technology published by Ching-Che Chung et al. in IEEE Proceedings of 2010 International Symposium on Circuits and Systems, All-Digital Time-Domain Smart Temperature Sensor With an Inter-Batch Inaccuracy of −0.7 DEG C.-+0.6 DEG C. After One-Point Calibration published by Poki Chen et al. in IEEE Transactions on Circuits and Systems, An Ultra Low Power 1V, 220 nW Temperature Sensor for Passive Wireless Applications published by Yu-Shiang Lin et al. in IEEE Custom Integrated Circuit Conference and a patent for an invention entitled Low-Voltage and Low-Power CMOS Temperature Sensor with application NO. CN102338669A, applied by Hong Kong University of Science and Technology on Feb. 1, 2012.
The above four intelligent temperature sensor technologies belong to an integrated temperature sensor circuit technology that is universally adopted in the industry for carrying out signal processing in the time domain. The types of integrated temperature sensors implemented on a semiconductor chip have common characteristics: first, a temperature acquisition and signal conversion element is adopted for converting measured temperature information into a time domain pulse signal related to pulse width and temperature, then a TDC (Time-Digital-Converter) is adopted for converting the time domain pulse signal into a multi-bit digital code stream output, and finally, a temperature reading is obtained according to the corresponding relation between the multi-bit digital code stream output and the temperature. FIG. 1 shows a basic structure of an existing time domain temperature sensor circuit, which comprises a PTAT (Proportional To Absolute Temperature) current generation circuit, a CTAT (Complementary To Absolute Temperature) current generation circuit, a first level-detection circuit, a second level-detection circuit, an XOR (Exclusive OR) gate and a counter, wherein the current output by the PTAT current generation circuit and the temperature are in direct proportion, and the current output by the CTAT current generation circuit and the temperature are in inverse proportion. The PTAT current generation circuit and the CTAT current generation circuit are respectively used for generating a current signal IPTAT in direct proportion with temperature and a current signal ICTAT in inverse proportion with temperature, and the current signal IPTAT and the current signal ICTAT are respectively used for charging for a first capacitor and a second capacitor. When the voltage of the positive end of the first capacitor reaches a threshold voltage of the first level-detection circuit, an output signal VP,D of the first level-detection circuit is overturned; when the voltage of the positive end of the second capacitor reaches a threshold voltage of the first level-detection circuit, an output signal VN,D of the second level-detection circuit is overturned. The output signal VP,D of the first level-detection circuit and the output signal VN,D of the second level-detection circuit are connected to two input ends of the XOR gate, and after XOR processing, a pulse signal PW in linear correlation to the pulse width and temperature is output. The pulse signal PW is connected to an enable end EN of the counter, the counter adopts a system clock CLK to carry out time domain sampling on the pulse width of the signal PW and convert the signal into a discrete digital pulse square wave signal in the time domain, and the temperature reading is obtained by the counting result for the discrete digital pulse square wave signal according to a known comparison relation.
The implementation manner has apparent deficiency in a system with insufficient frequency precision of a clock signal, namely, the change of the frequency of the clock signal, which is also called the change of cycle, can lead to the change of a quantization result of the pulse width in the time domain, so as to cause the reading error and the reading inconsistency of the temperature sensor. For example, in the application field of a passive RFID (Radio Frequency Identification) tag, a clock signal adopted by the temperature sensor integrated in a chip of the RFID tag is a carrier signal sent through an antenna at a transmitting terminal by a card reader and is extracted from received radio-frequency field energy by a clock recovery unit of the chip of the RFID tag, and therefore the temperature reading obtained from the counting of the clock signal is related to the carrier frequency sent by the card reader. According to the relevant norms and international standards of the field, the carrier frequency sent by the card reader is not a strictly precise frequency, but any frequency within the relatively wider frequency scope, and therefore the temperature readings of the temperature sensor in the same RFID tag read by different card readers have considerable difference and are apparently inconsistent. Even though output frequency indexes are same, the cycle of the clock signal extracted by the clock recovery circuit changes significantly with the change of circuit parameters, working environment and manufacturing process. And therefore the time domain temperature sensor implemented by the manner has lower precision, particularly for clock signals from different card readers, the reading consistency is very poor, thereby seriously limiting the application scope of the time domain temperature sensor.
For example, for a temperature sensor disclosed by a patent entitled Low-Voltage and Low-Power CMOS Temperature Sensor with application NO. CN102338669A, the working principle of the temperature sensor is shown in FIG. 1 and FIG. 2, wherein after the first capacitor is charged for a period of time TP,D, the voltage VP of the positive end of the first capacitor increases to a threshold voltage VP,TH, and a time delay signal VP,D with time delay TP,D is generated by the first level-detection circuit through the voltage VP; while after the second capacitor is charged for a period of time TN,D, the voltage VN of the positive end of the second capacitor increases to a threshold voltage VN,TH, and a time delay signal VN,D TN,D are generated by the second level-detection circuit through the voltage VN. The time delay signals VP,D and VN,D are respectively input into two input ends of an XOR logical gate, and the pulse signal PW in linear correlation with the pulse width and temperature is obtained through XOR logical operation. The pulse signal PW is output into an enable input end of a counter, so that the counter can count through taking the system clock CLK as the clock during high voltage-level TPW (pulse width of PW) of the pulse signal PW. When the counting result is K (K is an integer greater than or equal to 0), the pulse width of the pulse signal PW has K clock cycle(s), and then the temperature reading measured by the temperature sensor is obtained according to the preset corresponding relation between the quantization result K of the pulse width of the pulse signal PW and the temperature.
However, the system clock CLK of the temperature sensor disclosed by the patent entitled Low-Voltage and Low-Power CMOS temperature Sensor with application NO. CN102338669A directly adopts the clock recovered by the radio-frequency front end of the RFID tag from the field energy sent by a card reader, and therefore the output value of the temperature sensor is directly related to the carrier frequency received by the RFID tag. At present, the carrier frequency of the card reader is not strictly stipulated in the RFID industry standard, and therefore even the carrier frequency sent by the card readers produced in different batches of different manufacturers has considerably large difference, so as to lead to considerably large difference in the cycle of the system clock CLK recovered by the temperature sensor. And therefore even though the pulse width TPW is same (the temperature is same), when a system clock CLKA recovered by a card reader A is used, the counting result of the temperature sensor is K CLKA cycle(s), as shown in FIG. 2; while when a system clock CLKB recovered by a card reader B is used, the counting result of the temperature sensor is K-X CLKA cycle(s) (X is a natural number), as shown in FIG. 3. Above all, although the same temperature is measured by the same temperature sensor, different temperature readings can be read by different card readers, the temperature value is an absolute quantity; apparently, the temperature sensor has very poor precision and poor consistency and is not practical.
The above describes the problem that the precision and the consistency of the temperature sensor is compromised due to the cycle deviation of the clock signals input by different temperature sensors or in different application occasions. However, for the same temperature sensor, the cycle of the clock signal also possibly changes during one-time temperature measurement, as the cycle of the clock signal can change with the time fluctuation due to the factors of application systems, circuit noise, manufacturing process, and so on. For example, in the field of RFID, the fluctuation of the carrier frequency of the card reader causes the fluctuation of the frequency of the clock signal recovered by the passive RFID tag from the energy field, and the fluctuation of the frequency of the clock signal, i.e. the fluctuation of the cycle, is possibly generated during one-time measurement. Similar to the above situations, during the measurement of the temperature sensor, the variation of the cycle of the clock signal can also affect the measurement precision and the consistency of the time domain integrated temperature sensor.